1. Field of the Invention
This invention relates to electronic circuits, and more particularly, to circuits for detecting aging in electronic circuits.
2. Description of the Related Art
Over the life an electronic circuit, the effects of aging may have an impact on its operation. Factors such as operating time, voltage, and temperature may change one or more characteristics of various circuit elements. For example, the threshold voltage of a transistor may change over the operating life of an integrated circuit (IC) in which it is implemented. A change in the threshold voltage of one or more transistors may in turn require a change in the supply voltage supplied to the IC. Generally speaking, an increase in the absolute value of a threshold voltage of one or more transistors in an IC may correspond to an increase in the required supply voltage for correct operation.
One common degradation mechanism that manifests itself during the aging of transistors in electronic circuits is negative bias temperature instability (NBTI), which may apply to PMOS (p-channel metal oxide semiconductor) transistors. The affects on a PMOS transistor of NBTI over a period of time may cause an increase in the absolute value of the threshold voltage, along with a decrease in drain current and transconductance. As the absolute value of the transistor's threshold voltage increase over time, a higher supply voltage value is required to ensure that the circuit operates properly. A similar phenomenon, positive bias temperature stability (PBTI) may affect NMOS (n-channel metal oxide semiconductor) transistors. Another potential degradation mechanism is hot carrier injection (HCI), wherein electrons or holes may gain sufficient kinetic energy to overcome barriers between different portions of the silicon (e.g., the barrier between the silicon substrate and the gate dielectric). Higher operating temperatures may exacerbate HCI, as the higher temperatures may be associated with higher amounts of kinetic energy in the electrons/holes. Over time, HCI may degrade the gate dielectric of a transistor, increase its sub-threshold leakage current, and may also shift the threshold voltage.
In order to compensate for the effects of aging, IC's may be operated at a supply voltage that includes a guard band. For example, if the transistors of a given IC require a minimum supply voltage of 900 mV for correct operation, the IC may be operated at 1 V. Thus, as the degradation mechanisms discussed above cause an increase in the minimum required supply voltage, the guard band may provide protection against erroneous operation.